1. Field of the Invention
The present invention relates to a signal input device, and more particularly to a signal input device adapted to be used for leading signals received by an antenna into an electronic appliance such as a transformerless type television through a coaxial cable or the like.
2. Description of Prior Art
Generally, in a transformerless type television, signals received by an antenna are led into the television through a signal input device containing an electrical circuit as shown in FIG. 5. An input terminal 45 is connected to e.g. a coaxial cable 47 in the antenna side, and an output terminal 46 is connected to e.g. a coaxial cable 48 in the television side. Between the terminals 45 and 46, a CR parallel circuit 49 with a discharge gap is provided for a signal cable, and a CR parallel circuit 50 is provided for a grounding cable. The terminals 45 and 46 are insulated from each other in a low frequency range (e.g. 60 Hz) and coupled each other in a high frequency range.
When a television with such a signal input device is set in a strong electric field (grounding of the wire from the antenna to the television is secure), a coaxial cable extending from the signal input device to the tuner and a chassis are exposed to a strong direct wave, and a ghost image appears on the screen of the television. In order to solve this problem, Japanese Utility Model Laid Open Publication No. 62-58931 discloses that the coaxial cable of the signal input device is inserted in a hole of a ring ferrite core. This structure permits having little insertion loss but not sufficiently preventing the ghost image.
Japanese Utility Model Laid Open Publication No. 49-86753 discloses that a balun is provided in such a signal input device. This structure permits preventing the ghost image sufficiently but causes great insertion loss. Accordingly, when the television is set in a weak electric field, noise figure becomes bad. Further, in either of the structures above, when the television is exposed to a strong direct wave, great impedance occurs because a capacitor for the grounding cable in the signal input device does not have a large capacity. Accordingly, it is impossible to completely suppress a voltage induced by the direct wave.